西藏尼玛盆地古近纪湖相油页岩正构烷烃特征及其古环境与古气候意义

The characteristics of n-alkanes from the Palaeogene lacustrine oil shale in the Kanggale area, Nyima Basin, and their paleoenvironment and Paleoclimate significance

关键词:  正构烷烃  油页岩  西风环流  印度季风  尼玛盆地

Key words:  n-alkanes  oil shale  westerlies  The Indian monsoon  Nyima Basin

摘要:

尼玛盆地地处青藏高原中部特殊区域，自新生代以来沉积了大套连续的湖相暗色油页岩，其富含生物分子化石，对重建青藏高原古气候和古环境具有重要意义。本文通过对尼玛盆地古近系牛堡组康嘎勒剖面正构烷烃及相关参数分布特征进行研究，结果显示，剖面油页岩正构烷烃分布从底到顶由单峰前峰型向单峰后峰型过渡，具2个旋回周期；碳优势指数相对稳定，但TAR、ACL、P_aq及nC_≤21/nC_≥25变化较大，表明湖泊沉积有机质由内源为主的低等藻类向高等水生植物和外源陆生高等植物转变，且区域气候环境由温凉湿润向炎热干旱逐步演化。综合分析认为，尼玛盆地在始新世的气候变化在很大程度上是受西风环流和季风系统的共同影响。且西风环流和季风的强度变化与全球气候存在一定的关系。即全球变暖时，西风环流势力减弱，西风环流和残余洋水汽为高原中部提供的有效降水减少，气候相对干旱；而在全球降温时，西风环流和印度季风增强，残余洋水汽的叠加又为高原中部提供了充足的有效降水，气候湿润。

Abstract:

Continuous lacustrine dark oil shale with numerous biomolecular fossils were deposited in the Nyima Basin on the central Qinghai-Tibet Plateau since Cenozoic, It is of great significance to reconstruct the paleoclimate and paleoenvironment of the Qinghai-Tibet Plateau. By analyzing the distribution characteristics of n-alkanes and related parameters of the Palaeocene Niubao Formation in the Kanggale section, the results show that the distribution of n-alkanes in the oil shale varies from bottom to top from a single peak type to a single behind peak type with two cycles; The CPI is relatively stable, but the TAR, ACL, P_aq and nC≤₂₁/nC≥₂₅ are more variable, indicating a shift in the sedimentary organic matter of the lake from endogenous-dominated lower algae to higher aquatic and exogenous terrestrial higher plants, and a gradual evolution of the regional climatic environment from cool and humid to hot and arid. To sum up, we believe that the climate change in the Nyima Basin during the Eocene was mainly influenced by westerly circulation and monsoon systems, which is related to the global climate. During periods of global warming, there is less effective precipitation from the central plateau (provided by westerly circulation and moisture from the residual ocean) and the climate dries up as westerly circulation weakens. Instead, the intensification of westerly circulation and Indian monsoons enriched the central of the plateau with repeated effective precipitation, resulting in a wetter climate.